Now moving check execution into AccountManager, and _create_user() is gone,
replaced by more clearly structured and modularized code.

Registration checking for requests from the admin panel is re-activated and
minor improvements related to RegistrationError processing done too.

Note: The test, that detected admin requests to skip additional checking of
existing permissions for each new username, has been done differently.
In the future each check has to decide on its own, like this is done in theUsernamePermCheck now.

New unit tests helped me to recognize an unreported bug in the test about
effective configuration of email address verification.
Because if_enabled always returns something for a class, the email address
checking was unconditionally mandatory regarding EmailVerificationModule,
even if the module had been disabled. New check uses is_enabled instead.